The present invention is directed to a method of performing handoff procedures in a cellular communication system and, more particularly, to a method of prioritizing handoff procedures in a cellular system to reduce handoff failure.
A typical cellular communication system comprises a plurality of cells, each of which accommodates a number of mobile units within a predetermined geographical area. Each cell includes a base station which assigns to each mobile unit located in the cell a different communication channel for transmitting and receiving signals. Handoff procedures govern communications when a mobile unit leaves a first cell and enters a contiguous area associated with a second cell. When the mobile unit leaves the first cell, the communication channel assigned to the particular mobile unit is relinquished by the first base station, also referred to as the "old" base station, and a new communication channel is assigned to the mobile unit by the second base station, also referred to as the "new" base station.
A problem in performing handoff procedures is the availability of communication channels in the new cell. Typically, an overlapping region, also referred to as a handoff region, can be thought to exist between each pair of adjacent cells. In the handoff region, either base station may control signal transmission to and from the mobile unit. However, by the time the mobile unit exits the overlapping region, the new base station associated with the cell in which the mobile unit has entered must be able to facilitate continued signal transmissions by assigning a communication channel to the mobile unit. Many times, more than one mobile unit may be seeking to enter a new cell in which less than the necessary communication channels are available. As such, some of the mobile units may not be assigned communication channels by the new base station, resulting in a loss of signal transmission. Such a condition is referred to as handoff failure.
In some known cellular systems, the mobile unit continually monitors the signal strength of the base stations located in neighboring cells by monitoring a pilot signal transmitted by each base station. When the strength of the pilot signal exceeds a certain threshold, a handoff occurs. In other cellular systems, the base station monitors the signal strength of the mobile unit's signals to determine when handoff should occur. These methods tend to assign available channels to the mobile units in the order in which the handoff requests are received. Factors, such as the velocity at which the mobile unit is entering a handoff region, are not taken into account. As a result, a situation may arise in which a first mobile unit enters the handoff region and is travelling at a slower speed than a second mobile unit. If the new base station has only one channel available, according to the above-described method, the first mobile unit will be assigned the channel. If the second mobile unit exits the handoff region prior to another channel becoming available, the second mobile unit will no longer be able to transmit to the old base station associated with the cell it is leaving, resulting in handoff failure and a complete loss of service to the second mobile unit. Such a situation may arise even if the mobile units are travelling at the same speed because the second mobile's trajectory of motion, i.e. direction of travel, through the handoff region caused it to exit the handoff region earlier than the first mobile unit.
Cellular communication services is a very competitive business, and any invention that would reduce unintended service cutoffs to the customer would be of great benefit and value.